The present invention relates to a valve shifting mechanism for a brass instrument, and more particularly relates to a valve shifting mechanism with easily exchangeable parts and which produces a level of noise when operated and is therefore well suited for a trumpet or a like instrument.
Conventionally two types of valve shifting mechanisms have been used in trumpets to change the tonal pitch of the sound produced by the trumpet by changing the tube length of the trumpet in a stepwise manner. One such value is called an under spring type in which a spring for urging a piston tube is arranged under the tube. The other is called an upper spring type in which the spring is arranged above the tube.
One example of the conventional under spring type valve shifting mechanism includes a valve casing in the form of a tube open at both ends. This valve casing longitudinally accommodate a piston tube and a piston spring. The valve casing is provided with four radial bores spaced apart from each other both longitudinally and circumferentially for connection with the main tube and insert tubes. A longitudinal groove is formed in the inner face of the upper end of the valve casing in order to guide a valve guide. A top cover is screwed to the upper opening and a bottom cover is screwed to the lower opening of the valve casing, respectively. The piston tube has an outer diameter equal to or slightly smaller than the inner diameter of the valve casing to permit piston tube to slide smoothly in the valve casing without permitting air leakage. The piston tube is internally provided with three radial tubes corresponding to the radial bores in the valve casing. A piston rod seat is fixedly inserted into the upper opening and a spring seat is also fixedly inserted into the lower opening of the piston tube. The piston spring is interposed between the piston rod seat and the spring seat in order to urge the piston tube upwards.
A screw hole is formed through the center of the piston rod seat and a male screw formed at the bottom end of a piston rod is screwed into this screw hole. The top end of the piston rod projects upwards beyond the top cover of the valve casing and is provided with a presser cap.
The valve guide blocks the piston tube against axial rotation of the piston tube relative to the valve casing while allowing its longitudinal displacement. The valve guide is provided with a monolithic projection adapted to engage a recess formed in the top face of the piston rod seat near its periphery. A set screw is used to fix the engagement so that the projection is freely slidable in the longitudinal groove in the valve casing. A presser felt is interposed between the presser cap and the top cover and a piston felt is interposed between the top cover and the piston rod seat for absorption of mechanical shocks when the valve is operated.
The piston tube is normally held at the uppermost position by the piston spring so that the piston rod seat abuts the piston felt. When the presser cap is manually depressed under this condition, the piston tube moves downwards in the valve casing against the force of the piston spring and, depending on the extent of the movement, one of the radial tubes in the piston tube aligns with one of the four radial bores in the valve casing, thereby changing the valve length.
In this conventional valve shifting mechanism, the valve casing, the piston tube, the valve guide and the piston rod seat are all made of metal with the result that, harsh noises are generated due to frictional contact of the valve casing with the valve guide when the projection of the valve guide slides in the longitudinal groove in the valve casing.
Since the recess for the valve guide and the screw hole for the set screw have to be formed in the piston rod seat, a great deal of complicated labor is required to produce and assemble the piston rod seat. The same is true for the production of the piston tube since a groove has to be formed in the top end of the piston tube to accept the piston guide.
In another example of the conventional valve shifting mechanism, a metallic valve guide in the form of a ring is fixed atop a piston rod seat by means of a piston rod and a bent piece monolithically to the periphery of the valve guide is brought into engagement with a groove formed in the valve casing. No set screw is needed to fix the valve guide since the valve guide is fixed to the piston rod. Since, however, the valve guide is made of metallic material, valve shifting still generates harsh noises. Once a screw on the piston rod is inserted a center hole in the valve guide, the distance between the center of the piston rod and the bent piece is fixed. As a consequence, piston rods of different diameters can no longer be used, thereby seriously hampering apart exchangeability. In addition, the groove for the bent piece has to be formed in the top end of the piston tube, thereby making production complicated.
In the other example of the conventional valve shifting mechanism, a valve guide in the form of a pin is attached to the periphery of the top end of a piston tube and the top of the valve guide is brought into engagement with a groove formed in the valve casing. No set screw is needed as the valve guide is inserted by force into the piston tube. However, such force insertion tends to cause deformation of the piston tube. Since a metallic guide is used, shifting also generates harsh noises.